The present invention relates to method and apparatus for exposing a fine pattern such as a semiconductor to light, and more particularly to method and apparatus of light exposure which uses a short wavelength light for focusing and light exposure, detects an alignment mark on an article to be exposed to the light by a light having a longer wavelength than that of the exposure light through an exposure focusing system.
As a semiconductor circuit was miniaturized, a wavelength of a light used for the light exposure to a circuit pattern has been changed from a g-ray to an i-ray, and a light exposure method which uses an excimer laser beam has been introduced in Proceedings of SPIE-The International Society for Optical Engineering, Volume 633, 1986.
When the excimer laser beam is used, a wavelength thereof is one half of that of a visible band (450 nm .about. 700 nm) and several kinds of glass materials are required for exposing reduction lenses (hereinafter exposure lens and reduction lens are used synonymously). As a result, it is not possible to pay attention to a focusing characteristic to a wavelength other than the exposure light wavelength.
For example, in a reduction lens for a krypton fluride (K.gamma.F) excimer laser (wavelength 248.5 nm), a focusing system comprises quartz glass or a combination of quartz glass and fluorite.
When the above focusing system is used with an alignment light which is a visible light such as argon (Ar) laser (wavelength 515 nm) or helium-neon (He-Ne) laser (543.5 nm, 633 nm), it is difficult to focus an alignment mark on a wafer onto a reticle with a high resolution.
In the prior art apparatus, a focusing system for the registration mark (off-axis alignment system) is used separately from the reduction lens in order to effect alignment.
In the above method, however, it is difficult to maintain an optical center of the reduction lens and the off-axis alignment optical system in a precision of less than an alignment precision (approximately 0.1 .mu.m).
In other prior art light exposure technique, an ultraviolet ray (or far ultraviolet ray) which is the same or close to the wavelength of the exposure light is used together with the reduction lens to effect the alignment.
In this method, however, a depth of focus of the reduction lens is further shortened and it is necessary to form a multi-layer structure or a thick coating in order to make a wafer flat. Accordingly, it is difficult to irradiate an alignment light which has the same wavelength as that of the exposure light onto an alignment mark below a resist layer and detect a reflected light therefrom.